That's a heavy question, and frankly, the word 'top' is tricky because what one person finds compellingly transgressive, another might find gratuitous. I gravitate toward stories where the taboo is a vehicle for exploring broken psychology rather than just a shock tactic. Sierra Simone's 'Priest' comes up a lot, and while the priest/parishioner dynamic is the hook, the real weight is in the crisis of faith and the slow-burn erosion of a man's vows. It's less about the act and more about the spiritual and emotional torment that makes it feel forbidden.
On a much darker note, books like 'Captive in the Dark' by C.J. Roberts or the 'Kings of Quarantine' series delve into non-consent and captivity. The complexity there isn't in justifying the actions, but in the unnerving, obsessive relationship dynamics that emerge from extreme power imbalances. You're not supposed to feel comfortable; you're watching a car crash of trauma bonding. It's not for everyone, and I often need a palate cleanser afterwards, but the emotional intensity can be weirdly magnetic when done with some psychological depth.
For a different flavor of 'forbidden,' I'd point to age-gap or guardian/ward stories where the tension is built on a foundation of care that morphs into something else. 'Birthday Girl' by Penelope Douglas plays with this, setting up a living situation that constantly tests boundaries. The taboo isn't just the age difference, it's the betrayal of a trust-based relationship. The best ones make you feel the guilt and hesitation alongside the attraction, so the eventual crossing of the line hits with real consequence.